Magnetic data storage media is useful in the storage of large amounts of data for computer systems. The data or information is stored as a series of magnetic field transitions oriented on a magnetic recording surface. The transitions are placed on the surface by a magnetic transducer commonly referred to as a head. The transducer converts electrical energy into a magnetic field, the polarity of which is switched according to the information to be recorded. The magnetic field causes magnetization to remain in the media after the field is removed. The information is stored as binary information in the polarity reversals, or transitions, remaining in the media. The transducer used with magnetic media may also act as a detector to detect data stored as magnetic transitions. The transducer senses a magnetic field emanating from the magnetized media. The sensed magnetic field is converted into an electric signal which is different depending on the polarity of the magnetic field. Information is then decoded from the electric signals. When the transducer places information on the recording media, the transducer is said to have written data to the storage media. When the transducer detects previously written data on the media, the transducer is said to have read the storage data. In general, systems for storing and retrieving information from magnetic media may employ a single transducer to both read and record data on the media, or they may employ dual transducers, one to read and one to write.
The recording media is flat (typically information is recorded on both an upper and lower surface of the media), circular, and thick enough to attenuate position deformations caused by external forces such as clamping. The center hole in the media is typically called a hub. The hub is where the recording media attaches to a motor, through an axis, which provides power to rotate the recording media. A transducer or head is flown on either side of the rotating surface of the recording media. The head is flown because when the recording media rotates, it creates air movement under the head which physically lifts the head off the surface of the recording media. The head remains close enough to the surface so that the magnetic field generated by the write transducer establishes magnetic transitions on the recording media surface or close enough so that a magnetic field in the media is sensed by the transducer. The transducer or detector is placed near the surface of the recording media by a mechanical means such as an arm or lever. The arm or lever moves the write transducer or detector radially (with respect to the circular media) in and out from the hub to the edge (also called rim) of the recording media as the recording media rotates about the hub. The write transducer writes data onto the recording media in tracks concentric with the hub of the recording media as a result of the radial motion of the transducer and the rotation of the recording media. A track is an area which forms a concentric band, having a width which is generally larger than the width of the write transducer, about the hub of the recording media.
Data is read from the recording media using the same motion of the detector with respect to the recording media as when the data is written. However, in order to read previously written data, the read transducer, or detector, must be positioned over the tracks on the recording surface where the writing transducer was to have written the data. This is important because the detector cannot sense the distance to the optimum data reading position from any one position. Positioning the detector over the recording area generally involves a rough positioning means and a fine positioning means. The rough positioning means places the detector over the tracks. Tracks have a width which is greater than the area on which the data is written. The fine positioning means is typically separate from the rough positioning means and operates to place the detector over the width of the track in which data is written, rather than simply over any part of the track.
Fine positioning means are especially useful in dedicated servo drive systems and sector servo drive systems having dual transducers for reading and writing. Sector servo drive systems have a fine positioning means which consists of writing predefined patterns of signals into reserved areas of tracks on the recording media and then reading back that same predefined pattern of signals. The objective is that by comparing the sensed signals to the predefined pattern of signals, the detector position with respect to the written track position can be modified to produce the most accurate reading of the data. Signals are used instead of data because some relationship between what is detected and the detector position must be established in order to adjust that relationship. Signals having varying characteristics such as frequency or amplitude with respect to position on the track can provide that relationship.
When the detector is the same as the write transducer (called a single element head) in the sector servo drive system, there is static misalignment of the one transducer with respect to the data due to mechanical movement. The position of the head is affected by temperature, tower tilt, and other factors which change with the movement of the recording media. However, when data is written onto the media and subsequently read from the media by radially moving the transducer to the same position as it was when the transducer wrote the data, the servo system ensures that the detector will correctly read the data. This is because, even if there is a difference in where the detector should be and where it is when data is written due to errors in the mechanical arm means which moves the transducer, that same error will exist when the detector attempts to read.
In contrast, when the drive system (whether sector servo, dedicated servo, or other type) has one transducer which writes data and one which reads data (detector) on the same mechanical arm, this system is called a dual element head system. A dual element head system has manufacturing tolerances in the alignment of the write transducer to the detector which significantly effect the ability of the detector to accurately read the data. This is because, when the mechanical arm means moves the detector and places it in the same radial position used when the write transducer wrote the data, the static misalignment is large enough to allow the detector to have difficulty sensing the magnetic field transition. As a result, even though the sector servo drive system moves the detector to the same radial position as was used when the data was written, the drive system may still not sense data correctly. Therefore, some fine position compensation to the detector position with respect to the write transducer position must be made when the mechanical arm is returning the detector to the position of the previously written data.
Fine positioning means are also useful in dedicated servo drive systems. A dedicated servo system is one in which there are at least two sets of recording media, transducers, and detectors. The first recording media has a permanent pattern of data written on it which can be read by a tracking detector dedicated to that recording media. The tracking transducer is mechanically linked to the write transducer and detector dedicated to the second recording media. This linkage is intended to maintain a stable relative position between the tracking detector and the the transducer and detector of the second recording media. Therefore, data written to the second recording media can be located by knowing the position of the tracking detector when the data was written. This position is indicated by the pre-recorded data already on the first recording media.
The problem with dedicated servo systems is that, in addition to the static misalignment of two different transducers on the same arm, mechanical and thermal effects from the operation of the disk drive introduce error into the position relationship between the tracking detector and the second recording media detector. Dedicated servo systems have one coarse positioning arm in addition to the data arm. Therefore, any significant errors of the data head radial position in one arm with respect to the coarse dedicated servo arm will result in the data detector potentially missing data it should sense. Recording media in storage systems typically rotate at approximately 3600 rotations per minute although this speed can be much faster or slower. The fast motion causes thermal problems because the temperature of the media varies as a function of the radial position on the disk due to the speed of the rotating surface which varies as a function of radial position. Thermal gradients due to temperature changes and temperature shifts can cause mechanical movement of actuator arms, tilting of actuators, and general misalignment of other mechanical parts. This movement causes errors in the head position. These errors are slowly time varying over the operation of the drive system and are large enough relative to the track width to cause position errors for reading of the previously written data. The movements manifest themselves as increasing the error rate of the drive system in reading data over the time the drive system is in operation.
Optical storage media is similar in function to the magnetic media. Data detectors must be placed adjacent the rotating storage media with such precision that the read error rate is sufficiently low. Optical storage media differs in that the optical sensors detect different physical phenomena than magnetic sensors. Specifically, the elements for writing optical data produce the presence or absence of light, rather than magnetic field transitions, which the detectors must sense. However, the detectors still have a mechanical means for being placed adjacent to the data. The mechanical means is still subject to thermal and mechanical error as in the magnetic media systems for the above cited reasons. Additionally, the mechanisms for keeping track of the position of the optical detectors, such as dedicated and sector servo drives, are similar to magnetic media. Therefore, the solutions to the problems encountered by magnetic media in correcting for detector position error are generally applicable to optical media as well.
The prior art has used several techniques for developing a fine positioning means to correct for the relative position error of the data detector or transducer with respect to the data tracks. A first technique was to define a thermal test area or track outside of the data recording area. A reference signal, which provides different frequency signals depending on the radial position of the transducer, is written to each half of the test track. The data detector periodically returns to the test track to read the reference signal. The fine positioning means compensates the position of the transducer until the frequency of the reference signal sensed by the transducer is the same as that previously sensed. The problem with this type of technique is that it requires circuitry not already in the drive system, in the form of a frequency discriminator, to compensate for the transducer position. Extra circuitry is expensive and consumes critical space in compact drives. Additionally, the servo compensation is limited in accuracy to the linear function of the frequency of the reference signal across the track. Given the dimensional sizes of tracks and errors, this linear function is not sensitive enough in many cases.
A second method used by the prior art to compensate for the position error of the detector is to reserve sections of individual data tracks for writing special patterns. In particular, the special patterns would return analog signals having a particular amplitude depending on the position of the transducer with respect to a reference point. The reference point would be the center of the written track, and the signals would be written onto each of the opposing halves of a written track. When the read transducer passes over these signals, the position of the read transducer will be compensated until the amplitude received from the signals on each half of the track is equal. The difficulty with this technique is that, as above, extra circuitry in the form of analog detection circuitry is required. Additionally, the space on the recording media that is reserved for the special signals takes away from the available space for data storage.